It has been known in sewing machines from U.S. Pat. No. 4,583,419 that a balance wheel made of plastic, which also acts as a drive wheel at the same time, is detachably fastened to the arm shaft of the sewing machine and that it is secured against rotation. The balance wheel has, among other things, a hub and a plurality of ribs radially protruding therefrom. A transversely protruding screw, which has a cylindrical collar and a truncated cone-shaped head, is used to secure the balance wheel against rotation. The collar of the screw engages an axially extending recess of the hub, while the head of the screw is accommodated between two adjacent ribs. To reliably transmit the torque, the flexural strength and the wear resistance of the said two ribs is increased by a plug-in wedge made from a steel plate such that the head of the screw is supported on two corresponding legs of the plug-in wedge. A spring bent in the shape of a U, whose middle part is located in an annular groove of the arm shaft and whose bent legs engage correspondingly shaped slots of two other ribs of the balance wheel, is used to secure the balance wheel axially.
This type of fastening is highly complicated due to the use of a rather large number of specially manufactured and therefore not commercially available components. Even though it would be possible to eliminate the plug-in wedge, which increases the flexural strength and the wear resistance, if this prior-art type of balance wheel fastening were also used for manually operated balance wheels, which do not need to transmit any motor torque, at least the specially manufactured U-shaped spring for axially securing the balance wheel should still be there.